LONDON — British marine police and special forces, in fast boats and helicopters, stormed aboard an oil tanker in the English Channel and arrested seven stowaways who had threatened the crew, authorities said Monday.

The Liberia-flagged vessel Nave Andromeda was scheduled to dock Sunday at the port of Southampton on England’s southern coast when reports emerged that the 748-foot vessel was steering erratically out in the channel, one of the most heavily trafficked waterways in the world.

The Defense Ministry initially called the incident a “suspected hijacking.” Later reports said the seven stowaways, believed to be Nigerians seeking asylum in Britain, became threatening when they were ordered into a locked cabin.

British special forces responded to a request for help from police on Sunday evening. “Armed forces have gained control of the ship and seven individuals have been detained. Police investigations will now continue. Initial reports confirm the crew are safe and well,” the Defense Ministry said in a statement.

The BBC said the special forces “ended a 10-hour standoff which started when stowaways on board the ship reportedly became violent.” The special forces descended to the ship’s deck from helicopters hovering above.

According to police, the crew took shelter in a secure area aboard the ship called the “citadel.” The master of the ship remained on the bridge, the BBC reported.

The Hampshire police, in a statement, said the incident began Sunday morning, when “concerns were raised to police for the welfare of crew on board the vessel,” just six miles off the Isle of Wight.

“The vessel had been traveling in the direction of Southampton, having sailed from Lagos in Nigeria. It was reported that a number of stowaways were on board, and they had made threats towards the crew,” police said. “Following a multi-agency response by police, with support from the military and other emergency service partners, seven people were detained by police.”

The BBC quoted Navios Tanker Management, operator of the crude oil tanker, as saying the ship’s captain feared for the safety of his crew “due to the increasingly hostile behavior of the stowaways,” who had “illegally boarded” in Lagos, Nigeria.

The stowaways became violent when the crew attempted to lock them in a cabin, the BBC reported.

Defense Secretary Ben Wallace said Sunday evening: “In dark skies, and worsening weather, we should all be grateful for our brave personnel. People are safe tonight thanks to their efforts."